1. Field of the Invention
The present invention relates to an electrophotographic photocunductive composition. In greater detail, it relates to a laminate type electrophotographic light-sensitive material comprising a polyvinylpyridine resin as a binder of a charge generation layer.
2. Development of the Invention
Generally, essential requisites for electrophotographic light-sensitive materials include charge retention capability, high sensitivity, cycle stability, pre-exposure effect resistance (i.e., a property for how fast the surface electric potential of the light-sensitive material when electrically charged after high exposure returns to that when electrically charged before high exposure), dielectric breakdown resistance, friction resistance, solvent resistance, chemical stability (to ozone, etc.), lack of toxicity, storate stability, good developing capability, good transfer capability (i.e., a good toner transfer property from an electrophotographic light-sensitive material to an image-retention material (e.g., paper)), ease of cleaning, spectral sensitivity, low cost, etc.
Recently, laminate type electrophotographic light-sensitive materials have been actively studied in order to provide material satisfying the above requisites, and a number of materials been developed; however, almost all of them have a construction where an electric charge generation layer and an electric charge transport layer are laminated in this order on an electrically conductive base such as aluminum or copper.
The electric charge generation layer of such laminate type electrophotographic light-sensitive materials can be classified into two types. Firstly, vacuum evaporation types obtained by vacuum evaporation of an inorganic substance (such as selenium, seleniumtellurium, cadmium sulfide, etc.) or organic substances comprising dyes or pigments such as various azo pigments, phthalocyanine pigments, polynuclear quinone pigments, indigoid pigments, perylene pigments, quinacridone pigments, pyrylium dyestuffs, thiopyrylium dyestuffs, cyanine dyestuffs, squalidium dyestuffs, triphenylmethane dyestuffs or xanthene dyestuffs. A second class is the pigment dispersion types which are produced by applying a dispersion of fine particles of the above-described inorganic or organic substances in a binder resin such as a polyester resin, acryl resin, polystyrene resin, vinyl acetate resin, vinyl chloride resin, polycarbonate, butyral resin, silicone resin, epoxy resin, melamine resin, urethane resin, etc., or an electric charge generation layer obtained by applying a solution of a dye and the binder resin.
The electric charge transport layer is typically obtained by applying at least one electric charge transport substance selected from electron donating substances such as derivatives of pyrazoline, triphenylmethane, oxadiazole, carbazole, imidazole, oxazole, thiazole, etc., photoconductive polymers such as poly-N-vinylcarbazole, poly-9-vinylphenyl anthracene, etc., with a binder resin, or at least one electron accepting substance such as 2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitrofluorenone, 2,4,7-trinitro-9-dicyanomethylenefluorenone, 2,4,5,7-tetranitroxanthrone, etc., with a binder resin, to the electric charge generation layer together with an electrically insulating film forming material such as polyester, polycarbonate, etc. In the above regard, the addition of a small amount of the electron accepting substance to the electron donating substance, or of a small amount of the electron donating substance to the electron accepting substance, may be effected.
Essential requisites for the binder resin used for the electric charge transport layer include high electric insulating and good charging characteristics, high dielectric strength, high friction resistance, no change in charging characteristic by temperature and moisture change, freedom from lowering the electric charge transporting capability of electric charge transport substance, good compatibility with the electric charge transport substance and hard occurrence of crystallizing the electric charge transport substance, and the like. Suitable examples of the binder resin satisfying these requisites include resins conventionally employed, such as polyester resins, polycarbonate resins, acryl resins, etc. Photoconductive polymers which have themselves an electric charge transporting capability, such as poly-N-vinylcarbazole and poly-9-vinylphenyl anthracene can also be employed. These resins can be used alone or in combination with one or more other resins. Specific examples of the binder resins include those disclosed in U.S. Pat. No. 3,850,630 (col. 8, line 67 to col. 9, line 18).
Generally, in electrophotographic light-sensitive materials which are used repeatedly, considering physical strength, the electrically conductive base, the electric charge generation layer and the electric charge transport layer are superposed (in this order) to form a laminate. It is also possible, however, to form a laminate by superposing in the order of: electrically conductive base, the electric charge transport layer and the electric charge generation layer.
The present invention relates to laminate type electrophotographic light-sensitive materials comprising at least one electric charge generation layer and at least one electric charge transport layer comprising at least one charge generating substance and at least one binder therefor. In the case of laminating the electric charge transport layer on the electric charge generation layer, the electric charge transporting substance is dissolved in an organic solvent composed of cyclic ethers such as tetrahydrofuran, etc., ketones such as methyl ethyl ketone, methyl isobutyl ketone, etc., or aromatic hydrocarbons such as benzene, toluene, xylene, etc., with a binder and the resultant solution applied to form a laminate. A solids content in the applying solution varies depending upon the applying method and kind of binder resin used, but generally is less than 30 wt%.
In the case of carrying out application as described above, the organic solvent dissolves the electric charge generating substance and the binder resin in the electric charge generation layer and, consequently, the electric charge generating substance is mixed in the electric charge transport layer to result in a remarkable deterioration in sensitivity.
Further, the electric charge generation layer is seriously degraded and loses coating film uniformity (the electric charge generating substance is not distributed uniformly), whereby coating film defects such as color spots occur.
Therefore, very important characteristics required for the binder resin for the electric charge generating substance include solvent resistance in the case of applying the electric charge transport layer in addition to dispersion stability for pigments, stability to dye solutions, storage stability, high sensitivity, cycle stability, pre-exposure effect resistance and good adhesion to the conductive base and the electric charge transport layer, etc. As a first method of improving solvent resistance, thermosetting resins such as urethane resins, melamine resins, etc., have been used; however, they generally have low sensitivity and cause desensitization when the electric charge generating substance is unstable to heat. Further, the coating film of the electric charge generation layer has the defects that it becomes brittle and cracks easily occur to cause defects of the coating film. Therefore, thermosetting resins are not preferred for use.
As a second method, water-soluble resins such as casein, polyvinyl alcohol, ethylene-acrylic acid copolymer, etc., have been used, but they have inferior dispersibility for electric charge generating substances, particularly pigments, and cause aggregation. Further, they have problems in that the sensitivity is low, and cyclic stability and storage stability, particularly moisture resistance, are poor. Therefore, the aqueous resins are also difficult to use.
A third method involves using binder resins for the electric charge generating substances which are thermoplastic and soluble in organic solvents so that the electric charge generation layer is not damaged by the solvent used for applying the electric charge transport layer. However, to date there have been discovered few resins which satisfy all requisites required for a binder resin for an electric charge generating substances.